1. Field of the Invention
This invention relates to a remote plasma enhanced chemical vapor deposition (RPECVD) apparatus and method for growing an epitaxial semiconductor layer.
2. Discussion of Background
Plasma enhanced processes have figured prominently in research efforts to lower process temperatures. In conventional plasma enhanced chemical vapor deposition (PECVD), the parent gas molecules are dissociated into precursor atoms and radicals which can deposit on substrates at lower temperatures than in thermal chemical vapor deposition. The deposition occurs at lower temperatures than purely pyrolytic processes because the plasma supplied energy to break chemical bonds in the parent molecules that would only be broken by thermal decomposition if the plasma were not present. Parent molecule dissociation is accomplished in the plasma through various processes involving collisions with electrons, ions, photons, and excited neutral species. Unfortunately, the precursor species are also subject to the same active environment which dissociated the parent molecules. This can lead to further dissociation or reaction of gas phase species to form more complicated radicals before the radicals can condense on the substrate In a low pressure, low power silane (SiH.sub.4) immersion plasma, Matsuda et al. Thin Solid Films 92,171 (1982), have shown using mass spectroscopy that there are a host of gas phase species. These species include H, H.sub.2, Si, SiH, SiH.sub.2, SiH.sub.3, SiH.sub.4, Si.sub.2, Si.sub.2 H, Si.sub.2 H.sub.2, Si.sub.2 H.sub.3, Si.sub.2 H.sub.4, and Si.sub.2 H.sub.5. The most dominant line in the mass spectroscopy is the SiH.sub.2 line, even though it is ony 12% taller than the SiH.sub.3 line and 125% taller than the Si.sub.2 H.sub.5 line. There is a wide spectrum of precursor species incident on the growing film. A further complication is that in conventional PECVD the substrate is immersed in the plasma region. This results in a large flux of charged species incident on the substrate during film deposition. The incident energies of these ions may be as high as 160 eV in some immersion systems (See Chapman, Glow Discharge Processes, John Willey & Sons, N.Y. 1980, Chap. 4). This can lead to ion implantation, energetic neutral embedment, sputtering, and associated damage This residual damage must be annealed out during growth if high quality epitaxial layers are to be produced. Thus, this damage imposes a minimum growth temperature, based on annealing conditions below which high quality material cannot be obtained. Thus, there are two major problems associated with conventional PECVD: adequate control over incident gas phase species, and ion damage as a result of the substrate being immersed in the plasma region.
RPECVD deposition of silicon nitride Si.sub.3 N.sub.4 and silicon SiO.sub.2 for gate insulators in (In, Ga) As FET devices has recently been disclosed by Richard et al. J. Vac. Sci. Technol. A3(3), May/June 1985 (pages 867-872). According to this reference, to deposit SiO.sub.2, for example, one reactant, O.sub.2, is excited in the plasma tube remote from the semiconductor substrate. The other reactant, SiH.sub.4, enters the reactor separately, near the substrate and is not excited to a plasma state. An important point is that one of the reactants, O.sub.2, bearing one of the component atoms of the SiO.sub.2, is introduced through the plasma tube. The process is thought to follow the following reaction model. Monosilane (SiH.sub.4) molecules interact with the metastable oxygen O.sub.x *(.sup.3 P.sub.j) flux resulting from the remote plasma. The lifetime of the metastable oxygen is quite long, allowing pathlengths of 1-2 meters in the RPECVD reactor using the SiO.sub.2 deposition parameters. (In contrast, the pathlength of a typical metastable excited noble gas specie, e.g. He*, used in the RPECVD epitaxial growth of semiconductor layers, according to the present invention, is 5-30 cm.) This interaction leads to disiloxane, (SiH.sub.3).sub.2 O, formation in the gas phase. On the heated substrate, disiloxane is further oxidized by excess metastable oxygen, O*. This oxidation removes H from the silyl groups, SiH.sub.3. Dehydrogenation is accompanied by oxygen bridging of silicon atoms originally bound in adjacent disiloxane molecules on the heated surface. An excess of the plasma excited species is used to drive the dehydrogenation of the silyl groups to completion, minimizing Si--H bonding. Silicon-poor films do not form; thus the process is stable. For this case, CVD can be thought of as a polymerization of disiloxane brought about by oxidation of the SiH bonds of the silyl groups.
Important features of the SiO.sub.2 process described by the above-noted Richard et al article are:
1) In the SiO.sub.2 process, O is activated by the plasma in the plasma generation region and becomes incorporated in the deposited layers.
2) The interaction between the reactive species existing the plasma generation region and the injected reactant results in the formation of the chemical groups.
3) The lifetimes and therefore the pathlengths of the reactive species exiting the plasma formation is quite long: for metastable oxygen the pathlength is 1-2 meters.
4) The dielectric material formed, SiO.sub.2, is an amorphous material and therefore has no long-range or crystalline order. For SiO.sub.2 deposition, metastable O* promotes the further oxidation of disiloxane adsorbed on the substrate surface, which reduces the surface of adatoms and enhances the formation of amorphous material.
Another prior art reference of interest is an article by Toyoshima et al, Appl. Phys. Lett. 46(6), 15 March 1985, pp 584-586, which describes a PECVD process to deposit hydrogenated amorphous silicon. However, the deposited a-Si:H films retain from 5-30 atomic percent hydrogen in the deposited layers, which is critical to the performance of a-Si:H, but disastrous if one is trying to grow epitaxial Si layers. No process used to deposit high quality a-Si:H films has proven successful in depositing epitaxial Si layers.